Where has the export model to Python script option gone in QGIS 2.6.0?
The "export to Python" option in the modeler has been taken out in the latest version. The modeler has been rewritten and there was not enough time to implement this feature. We have released the new version without it, considering that the improvements that this new modeler have are really worth it. The "export to Python" version will be available again in future releases
If you are still seeing that option, it means that QGIS is not picking the latest version correctly and is loading an older version of Processing. Removing the [home]/.qgis2/python/plugins/Processing folder (in case it exists) might help, since it will remove any other version of Processing that was installed using the plugin manager.
This functionality is available again in the 3.6 release. The release notes are documented here: https://qgis.org/en/site/forusers/visualchangelog36/index.html#feature-resurrected-model-to-python-script-functionality